The present invention relates to a system for controlling cooling water temperature for a water-cooled engine mounted on an automotive vehicle, and more particularly to a system which controls the cooling water temperature to a proper temperature according to conditions of engine temperature and engine operation.
In a conventional cooling water control system, a water pump is driven by the engine, so that flow rate of the cooling water is varied by the engine speed. Further, the flow rate of the cooling water is controlled by a thermostat which is provided in a cooling water conduit pipe connected to a radiator and is adapted to be opened when the cooling water temperature exceeds a predetermined temperature to permit the cooling water to flow to the radiator. Thus, temperature of the cooling water is controlled by the control of flow rate. Therefore, the cooling water temperature is not controlled in accordance with conditions of the engine temperature and engine operation. Accordingly, the cooling water is not kept to a proper temperature.
The flow rate in the cooling system of a conventional system is decided so as to satisfy the operation of the engine in full-load conditions such as uphill driving with the full open throttle operation. Accordingly, the flow rate preferably set for such a full-load condition is excessively high for an operation at a slow speed or a partial load. Although the cooling water temperature at low speed is also controlled by the thermostat, combustion chambers and cylinders wall of the engine is over cooled, because the water velocity is high. This causes decrease of thermal efficiency of the engine.
In order to eliminate such disadvantages, it is necessary to control finely the flow rate of the cooling water. Japanese patent application layed open No. 50-25951 discloses a control system which is intended to meet such a requirement. However, the system does not satisfy the warming up in cold engine operation and the cooling water temperature is not controlled according to engine operation.